Lithium complexes cyclization

The base-catalysed cyclization of oximinonitriles in the presence of lithium ions to form 4-aminoisoxazoles and 5-aminooxazoles proceeds via the anion of one of the oximinonitrile lithium complexes. ... 

The bromo-alkoxylation route for the synthesis of crown ethers (6,189 5,178) has been adapted to allow the preparation of substituted mono- and di-thia-crown systems (Scheme 49). Once again, one of these sequences uses the cyclization that is effected by a sulphonyl chloride and a base, as does a recently reported preparation of oxo-crown ethers e.g. Scheme 50 for the mono-oxo-series. Studies here imply activation of the carboxy- rather than of the alkoxy-terminus by the sulphonyl chloride. The ester functions reduce the complexing ability of these macrocycles with respect to normal crown ethers. The reduction of oxo-crowns to cyclic ethers, rather than to the diols that might be expected from reduction of a lactone, by LiAlHi has been studied the crown system seems to be necessary, and the involvement of lithium complexes has been suggested. 

An analogous type of lithium-induced zipper cyclization was observed with PAM 5, affording the mesolDL pair of helicenes (Scheme 11) [34]. PAM 5 also reacted with dicobaltoctacarbonyl to give a tetracobalt cluster in which only two of the triple bonds have been complexed [35]. 

Conjugate addition.s The key step in a stereocontrolled synthesis of the Lyth-raceae alkaloid lasubine II (4) is the conjugate addition of an alkylcopper complexed with BF3 to the N-acyl-2,3-dihydro-4-pyridone 1 to give the d.v-product 2 in 56% yield and >96% stereoselectivity. Hydrogenation in the presence of Li2C03 effects cyclization and deprotection of nitrogen to give the ketone 3, which is reduced stereoselectively by lithium trisiamylborohydride to the desired alcohol 4. 

A new synthesis of dibenzoboroles by reductive cyclization of arylboron dibromides opens up access to several derivatives of this ring system (Scheme 6) <1996JA7981>. MonoaryIboron dibromides 61 and 62 were prepared by reaction of the appropriate aryl lithium compound 60 with boron tribromide in hexane. Reductive cyclization of arylboron dibromide 61 with an excess of lithium metal in diethyl ether gave bislithium dibenzoborole complex 63. At the time of writing, compound 63 was the first dibenzoborole dianion to be structurally characterized by X-ray... 

A similar [4 + 2] cyclization between 74 and CP was also conducted with a CH2CI2 solution of AlLi3tris(binaphthoxide) (75) [87, 88]. Catalyst 75 has a saturated coordination number with respect to the aluminum atom, which thus no longer has vacant orhitals for further complexation with other incoming Lewis bases. Whereas the ee of the adduct was very low (16 % ee), this result strongly suggests the possibility that lithium ions in 75 serve as Lewis acids in non-polar solvents. 

A later modification of the original KObrich procedure entails the generadon of carbenoids in the presence of an added equivalent of lithium bromide salt (equadon 22). Appaiendy, the Lewis acid character of the added s t helps to stabilize the carbenoid by coordination widi the hdogen atom. The lithium salt is also implicated in Lewis acid complexation to the oxygen atom of the C=0 bond, and seems to assist in the cyclization step as well. 

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